Reflective optics equipped with a cooling system

Abstract

The invention relates to reflective optics (1) for transporting, transforming or correcting a light beam (19), in particular of the laser type, characterised in that it comprises a mirror (2) receiving the light beam (19), a primary cooling circuit made up of an intermediate chamber (6) of thermally conductive fluid (7) disposed against the mirror (2) at the rear thereof and a secondary cooling circuit made up of a heat sink (3) disposed against the intermediate chamber (6) of thermally conductive fluid (7), said heat sink (3) being either in the form of a cold mass cooled by convection or conduction, or in the form of a plate made of a material having good thermal conductivity, the heat sink (3) having a size and a shape equivalent to those of said reflective optics.

Description

FIELD OF THE INVENTION

[0001] The technical field of the present invention is that of reflective optics used in light beam systems, in particular of the laser type, as well as their means of cooling.

[0002] As is well known, it is necessary to use reflective optical structures to transport laser beams, transform their profile, focus them or modify or correct the wavefront of said beam.

[0003] There are many different types of lasers, both in terms of power, emission mode or type of amplifying medium used.

[0004] When the laser is a high average power laser, the reflective optics used undergo a high accumulation of energy which results in heating. This is problematic because such heating leads to deformation of the optics, which causes optical aberrations in the laser beam, as well as degradation of these same optics. It is then necessary to replace them at a frequency which results in a cost that is difficult for the user to bear.

[0005] Reflective optics must therefore be cooled from the moment when natural cooling by convection with the ambient air is no longer sufficient, which is the case when they are used in high average power lasers (typically 5 to 20 kW). This is all the more critical when the laser in question is in a vacuum, such as those used in scientific laboratories such as PetaWatt lasers with a high repetition rate, delivering an average power of 10 kW or more, or continuous or pseudo-continuous mode lasers used for example in the microelectronics industry with average powers of 1 to 30 kW. There are also applications in astronomy, particularly in the infrared range, and in space observation for large aperture telescopes whose optical qualities can be degraded by thermal inputs due to the observed scene (earth, luminous star, etc.). STATE OF THE ART

[0006] Several cooling systems are known and used to cool reflective optics used in lasers.

[0007] For example, the article "CVD SiC deformable mirror with monolithic cooling channels" by Kyohoon Ahn et al., published in Optics Express Vol. 26, No. 8, in 2018, describes a mirror cooling system composed of channels through which water circulates, the channels being formed in the thickness of the mirror itself and the mirror being actuated by piezoelectric actuators. This system therefore requires intervention on the mirror itself, which is always difficult to achieve.

[0008] US-4657358 describes a deformable mirror cooled by channels perpendicular to the mirror, each channel being included in the rod of each actuator and forming a small circular chamber around the head of each actuator. It is therefore a system consisting of localized cooling points limiting the cooling performance.

[0009] Patent US-4844603 describes a cooled flexible mirror having a cooling chamber between said mirror and its support, the cooling chamber being filled with the cooling liquid.

[0010] The article "A cooled deformable bimorph mirror for a high power laser" by Jun Ho Lee, published in Journal of the optical society of Korea, June 2006, describes a convection and conduction cooling system, with convection provided by an oblique embossment included under the reflection surface and in which water circulates as a coolant, and conduction provided by a metal coating layer included in the embossment.

[0011] The article "Deformable mirror for high power laser applications" by Libor Mrña et al., published in Proceedings of SPIE, the international Society of optical Engineering, January 2015, describes an actuator-deformable mirror, cooled by a pressurized coolant flowing along the back surface of the mirror through the hexagonal structure of the mirror surface.

[0012] The article "Cooled and uncooled single-channel deformable mirrors for industrial laser systems" by BS Vinevich et al., published in Quantum Electronics, 1998, describes a bias embossing system included in the mirror through which a cooling liquid circulates.

[0013] The article "Development of a unimorph deformable mirror with water cooling" by Zhengxiong Zhu et al., published in Optics Express, November 2017, describes a cooling system where the coolant circulates through the back side of the mirror into a cooling cavity. A cooling unit at the periphery of the mirror maintains the coolant at a constant temperature of 20°C.

[0014] US patent 2001 / 008469 A1 describes a mirror deformable under the action of a deforming member.

[0015] However, these different systems have shortcomings or disadvantages.

[0016] Indeed, systems for cooling mirrors with air or a cooling liquid on the back face of the mirror at the periphery do not produce cooling efficient enough to be useful in the case of high average power lasers. These cooling systems create a temperature gradient at the active surface of the mirror, which is detrimental to its optical quality.

[0017] As for the cooling systems using liquid circulating on the back face of the mirror, although they have excellent thermal efficiency, the pulsations of the pumping of the cooling liquid cause the appearance of dynamic optical defects in the laser beam.

[0018] Finally, cooling systems using the circulation of a coolant in a set of channels located either in the thickness of the mirror or against the rear face of the mirror, lead to a reduction in the optical qualities of the mirror due to optical aberrations appearing on the laser beam due to the thermal imprint of the cooling channels. These are also systems whose manufacture is complex and expensive. STATEMENT OF THE INVENTION

[0019] The present invention provides a system for cooling reflective optics used in observation devices, telescopes, high average power lasers and for optical devices sensitive to thermal effects, without suffering from the disadvantages and limitations present in the prior art.

[0020] The invention relates to a reflective optic according to claim 1.

[0021] The dependent claims define advantageous embodiments.

[0022] For a lower average power, it may be appropriate to replace the block comprising the channel with the heat transfer fluid by a cold mass cooled by natural or forced convection using fins made on the back of the cold plate for example and / or by conduction thanks to the thermal connection of the cold mass to the vacuum enclosure for example in the case of a vacuum installation.

[0023] A first advantage of the cooling system according to the invention is that it is adaptable to fixed mirrors and deformable mirrors. It is sufficient to provide holes through the heat sink for the passage of the actuators coming into contact with these deformable mirrors.

[0024] Another advantage of the invention is that the cooling system can be adapted to all mirror shapes: flat, parabolic, rectangular, circular, etc.

[0025] Yet another advantage of the invention lies in the fact that the cooling system can adapt to a wide range of laser beam power, the cross-section of which can vary between a few square centimetres and more than 2500cm 2< .

[0026] Another advantage of the invention is that the cooling circuit is not in contact with the mirror itself. Thus, pressure fluctuations in this circuit do not affect the geometry of the mirror and therefore have no consequences on its optical function.

[0027] Another advantage of the invention is the low thermal resistance between the reflective face of the mirror and the heat sink, which allows for very efficient cooling.

[0028] Yet another advantage of the invention is that the mirror can be made of a material with good optical properties (glass and glass derivatives or silicon) having average thermal conduction properties.

[0029] Another advantage of the invention lies in the correction of static deformations of the mirror under the effect of gravity by adjusting the pressure of the heat-conducting fluid in the cooling chamber and the expansion tank.

[0030] Another advantage of the invention lies in the fact that the cooling system allows heat exchange to take place over almost the entire surface of the mirror.

[0031] Yet another advantage of the invention lies in the fact that the cooling system is two-stage, the first stage being constituted by the heat-conducting fluid present in the cooling chamber and the second stage being constituted by the heat-transfer fluid circulating in the cooling channel present in the heat sink, the combination of the two stages producing an efficient and uniform cooling effect.

[0032] Yet another advantage of the invention is that since the dissipator is made of a thermally conductive material, its temperature is homogeneous and does not present thermal variations likely to disrupt the operation of the reflective optics. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Other characteristics, advantages and details of the invention will be better understood on reading the additional description which follows in relation to the drawings in which: [ Fig 1] represents a rigid reflective optic equipped with the cooling system of the invention; [ Fig 2 ] represents a deformable reflective optic equipped with the cooling system of the invention; [ Fig 3 ] represents a cross-section of the heat sink of the figure 2 along the X-X' axis, and [ Fig 4 ] represents a particular arrangement of fixed supports. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0034] The invention will now be described in more detail. As previously indicated, it involves cooling a reflective optic in order to avoid its deformation under the effect of heat to minimize optical aberrations.

[0035] On the figure 1 , a reflective optic 1 is shown in section, incorporating a mirror 2 which in this version is a plane mirror 2a which reflects an incident beam 19, for example of the laser type, towards another optical path.

[0036] The mirror 2 is fixed to a dissipator 3 by means of supports 4. The supports 4 define, in cooperation with one or more sealing parts 5, an intermediate chamber 6, located between the mirror 2 and the dissipator 3. According to the invention, the supports 4 are astatic supports which thus prevent the transmission of pressure fluctuations or movements between the dissipator 3 and the mirror 2 as well as between the intermediate chamber 6 and the mirror 2.

[0037] Thus, in a configuration of ten supports, one can provide for example seven astatic supports 4 and three fixed supports 20 as shown in the figure 4 This chamber 6 is filled with a heat-conducting fluid 7. The intermediate chamber 6 communicates with an expansion tank 8 which allows the pressure of the heat-conducting fluid 7 in said chamber to be adjusted.

[0038] The dissipator 3 is a support block on which the mirror 2 rests. It has an internal structure perforated with a cooling channel 16 in which a heat transfer fluid 9 circulates.

[0039] On the figure 1 , we see that the dissipator 3 has a contact surface equal to that of the mirror. It goes without saying that this dissipator can have a different surface, either higher or lower.

[0040] For a lower average power, it may be appropriate to replace the block comprising the channel 16 with the heat transfer fluid 9 by a cold mass cooled by natural or forced convention by means of fins made on the back of the cold plate for example and / or by conduction thanks to the thermal connection of the cold mass to the vacuum enclosure for example in the case of a vacuum installation. It is advantageous for said cold mass to have good thermal conductivity and good specific heat. By good thermal conductivity or good specific heat, we mean values like those provided by copper.

[0041] The cooled cold mass appears in the form of a metallic mass, made of copper for example.

[0042] The reflective optics 1 and the dissipator 3 rest on a support 10 integrated into the laser assembly, not shown.

[0043] There figure 2represents a reflective optic 1 constituted by a deformable mirror 2b associated with the cooling system according to the invention.

[0044] The actuators 11 are supported by the support 10. These actuators pass through the dissipator 3 and the intermediate chamber 6 to come into contact with the rear face of the mirror 2b via a movable head 14. The structure of an actuator is well known and it is not necessary to describe it more fully.

[0045] The heat sink 3 is provided with holes 13 allowing the passage of the actuators 11 with a certain clearance. Each hole 13 is closed using a sealing ring 15 to ensure that the heat-conducting fluid 7 is maintained in the intermediate chamber 6.

[0046] In the same way as in the case of a fixed reflective optic, the figure 2represents a mirror 2 behind which opens an intermediate chamber 6 filled with a heat-conducting liquid 7. The mirror 2 rests on the dissipator 3 by means of supports 4.

[0047] One or more sealing parts 5 border the intermediate chamber 6. As previously, the dissipator 3 comprises a channel for circulation of a heat transfer fluid 9.

[0048] It is understood that the set described in relation to the Figures 1 and 2 allows the heat to be distributed over the entire rear face of the mirror 2a or 2b using the intermediate chamber 6, avoiding any temperature gradient. When the temperature increases, the heat transmitted by the mirror 2a or 2b causes the liquid 7 to circulate in the chamber 6 and the expansion tank 8. On the other hand, the chamber 6 allows the deformations of the mirror 2a or 2b to be reduced due to the hydraulic inertia of the liquid 7.

[0049] The intermediate chamber 6 and expansion vessel 8 assembly makes it possible to compensate for and control the deformations of the mirror 2. By modifying the altitude of the expansion vessel 8 relative to the intermediate chamber 6, the resistance of the intermediate chamber 6 against the deformations of the mirror 2 is varied. The heat received by the mirror 2a or 2b is distributed uniformly due to the absence of hot spots.

[0050] Thus, the evacuation of heat received by the mirror 2a or 2b is ensured using the dissipator 3 in which the heat transfer fluid 9 circulates.

[0051] When the laser beam 19 is a high average power beam, the mirror 2 receives a high energy load which increases its temperature. This thermal load is communicated to the thermally conductive fluid 7 then to the dissipator 3 for the purposes of diffusion and uniform distribution of the thermal flux allowing cooling in a first stage.

[0052] This first diffusion step is followed by a second step where the dissipator 3 is cooled in turn by the heat transfer fluid 9 which circulates in the channel 16 formed in the dissipator 3.

[0053] It is therefore a two-phase cooling system. This system allows efficient and uniform cooling of the reflective optics in which it is integrated.

[0054] There figure 3 represents a cross-section of the dissipator 3 along the axis XX' made in its thickness at the level of the channel 16. It can be seen that the channel 16 describes a path delimiting a certain number of loops close to each other. Thus, the mirror 2a or 2b can be cooled uniformly over its entire surface.

[0055] The channel 16 runs through almost the entire surface of the dissipator 3, the fluid inlet point 17 of which is cleverly located in its central zone and the fluid outlet point 18 located at its periphery. It goes without saying that this configuration is preferred, but any other configuration can be adopted depending on the shape and dimensions of the mirror and the energy it receives.

[0056] The holes 13 for the passage of the actuators are distributed over the entire surface of the heat sink between the different loops.

[0057] Mirror 2 can be polished. It can also be coated or uncoated to reflect the laser beam. The reflective treatment can be a coating applied to the optics, either metallic or dielectric, or a combination of both, to achieve a reflectivity of 99% or more.

[0058] The mirror is alternatively made of a thermally conductive material, for example an easy-to-polish metal such as copper or silicon. It is also possible to use a silica-based substrate to obtain optimal optical qualities, but the cooling capacity will be lower.

[0059] The heat-conducting fluid 7 may be, for example, Galinstan and the heat-transfer fluid 9 may be water.

Claims

1. A reflective optics (1) for transporting, transforming or correcting a light beam (19) in particular of the laser type, characterised in that it comprises a mirror (2) receiving the light beam (19), a primary cooling circuit consisting of an intermediate chamber (6) of thermally conductive fluid (7) disposed against the mirror (2) at the rear thereof, and a secondary cooling circuit consisting of a heat sink (3) disposed against the intermediate chamber (6) of thermally conductive fluid (7), said sink (3) having either the form of a metal mass cooled by convection or conduction, or the form of a plate made of a material having a thermal conductivity with a value such as that provided by copper, the sink (3) having a contact surface area equal to or different from that of said reflective optics, the mirror (2) being attached to the sink (3) using astatic supports.

2. The reflective optics (1) according to claim 1, characterised in that when the sink (3) is in the form of a plate, said plate includes in its thickness a channel (16) in which a heat transfer fluid (9) is circulated.

3. The reflective optics (1) according to claim 1 or 2, characterised in that said mirror (2) is a deformable mirror equipped with actuators (11) on its rear face, each actuator (11) passing through the intermediate chamber (6) of thermally conductive fluid (7) and the sink (3) via an aperture (13).

4. The reflective optics (1) according to claim 3, characterised in that each actuator (11) is provided with a head (15) translationally movable in an aperture (13) provided in the sink (3), the thermally conductive fluid (7) circulating in said aperture (13) around the movable head (14).

5. The reflective optics (1) according to claim 4, characterised in that the aperture (13) is closed by a sealing ring (15).

6. The reflective optics (1) according to any of claims 2 to 5, characterised in that the channel (16) integrated into the heat sink (3) is in the form of a path delimiting loops close to each other, said channel (16) running along the surface of the sink and having an entry point (17) and an exit point (18).

7. The reflective optics (1) according to claim 6, characterised in that the entry point (17) of the heat transfer fluid (9) into the channel is located in the vicinity of the centre of the sink (3).

8. The reflective optics (1) according to claim 6 or 7, characterised in that the exit point (18) of the heat transfer fluid (9) out of the channel is located in the vicinity of the periphery of the sink (3).

9. The reflective optics (1) according to one of the preceding claims, characterised in that the mirror (2), the intermediate chamber (6) of thermally conductive fluid (7) and the heat sink (3) are mechanically associated to form an integral block.

10. The reflective optics (1) according to any of the preceding claims, characterised in that the thermally conductive material constituting the sink (3) is metal, preferably copper.

11. The reflective optics (1) according to any of the preceding claims, characterised in that the intermediate chamber (6) of thermally conductive fluid (7) is in connection with an expansion tank (8).

Images